In the field of network communications, it is known to implement passive measurement techniques at selected points in a communications network in order to monitor Quality of Service levels and diagnose faults that can occur from time-to-time in the communications network.
In this respect, it is known to deploy so-called “passive probes” at the selected points in the communications network. Such passive probes make measurements relating to network traffic travelling along one or more links in the communications network. Additionally, once collected, measurement data has to be communicated to, for example, a central monitoring station in the communications network for analysis and interpretation.
In order to convey the measurement data from a passive probe to the central monitoring station, US 2005/0083957 A1 proposes a low bandwidth channel formed by inserting packets into a high bandwidth packet stream. The packets are inserted at a predetermined interval, insertion causing latency that is recovered by minimising inter-packet gaps in the incoming high bandwidth channel. While the packets to be inserted are being transmitted, arriving high bandwidth packets are stored in an elastic buffer.
However, whilst the above technique provides a mechanism for achieving the low bandwidth channel in the high bandwidth packet stream, it is desirable to improve performance of the low bandwidth channel. In particular, the above-described technique relies upon the existence of sufficiently large “gaps” in the high bandwidth data stream that can be reduced to allocate bandwidth to accommodate transmission time for the low bandwidth channel. However, the above apparatus preserves traffic flow in the high bandwidth data stream as the traffic in the high bandwidth data stream is considered to be of greater importance than the traffic using the low bandwidth channel. This is an overriding principle to which operation of the above apparatus adheres. Hence, if insufficient gaps exist in the high bandwidth data stream, transmissions on the low bandwidth channel have to be halted due to lack of bandwidth until a sufficiently large gap occurs in the high bandwidth stream.
As a result of a temporary incapability to transmit on the low bandwidth channel, it is necessary to buffer the measurement data to be transmitted until the measurement data can be transmitted. Whilst the apparatus described above comprises the elastic buffer, there is a limit to elasticity of the buffer. To avoid running out of buffer capacity, the apparatus reaches a point where creation of new packets has to be temporarily halted until bandwidth becomes available to resume transmission of the measurement data. Alternatively, the apparatus discards packets already created (and in the buffer) in order to provide capacity in the buffer for newly created packets containing measurement data.